Digital cameras typically have two focusing methods available. One method is an auto focus mode and the other method is a manual focus mode where the user sets the distance for the object to be focused. The manual focus mode is used when the auto focus mode is incorrectly determining the correct focus for the object the user wishes to photograph. In the manual mode the user sets a distance and the camera typically moves the lens to a position that corresponds to the focus for that distance. FIG. 1 shows the distances used in the calculation to determine the proper lens location. Distance f is the focal length of the lens, distance s is the distance from the lens to the object to be focused, and distance s' is the distance from the lens to the image of the object. In the manual focus mode the user sets the distance s and the camera calculates distance s' using the formula 1/s'=1/f+1/s. Once distance s' has been determined the camera will position the lens for the correct focus.
The focal length of a lens is dependent on the focal length of each element in the lens and the spacing between the lens elements. The focal length of an individual lens element is dependent on the index of refraction of the material used for that lens element, the curvature of the two surfaces of the lens element and the thickness of the lens element. The index of refraction, the curvature of the surface and the thickness of the lens are dependent on the temperature of the lens element. The spacing of the lens elements is also dependent on the temperature of the lens system. This means that a change in temperature of the lens system will typically change the focal length for the lens system. When the temperature of the lens system in a digital camera changes without a corresponding change in the focal length used in the calculation to determine the correct lens position for a given object distance, the lens will not be positioned correctly for optimum focus.
Scanners are digital imaging systems that have a constant distance between the object and the lens, and between the image and the lens. When there is a change in temperature in a scanner the focal length of the lens changes. Typically scanner lenses are fixed in place and can not be adjusted for the change in focal length. When the change in focal length is known, the image can be digitally adjusted to compensate for the change in focus.
There is a need for a digital imaging system that can adjust for changes in temperature and determine optimum focus for a given object distance.